Hot on the heels of NASA's recent announcement that the subsurface ocean on Jupiter's moon Europa is one of the solar system's likeliest bets for alien life is more food for thought.
Saturn's giant moon Titan, larger than the planet Mercury, is also home to a subsurface ocean — only this one seems likely to contain as much liquid as all the world's oceans combined.
Earlier this week, NASA's Saturn-orbiting spacecraft, Cassini, swooped low across Titan's surface, making crucial gravity measurements. (A better understanding of the moon's gravity will reveal more about the size and composition of its subsurface sea.)
At its closest approach, the Cassini probe flew within 1978 kilometres of the distant moon's icy surface.
Although it is too soon to be certain of exactly what the ocean consists of, astrophysicists can hazard a shrewd guess. "The data will take months to analyse," says Cornell University astronomer Jonathan Lunine. "But we can be very certain from past flybys that there is a liquid layer underlying the crust of Titan."
Dr Lunine is part of a team of scientists, led by Luciano Iess of Italy's Sapienza University, who previously studied how Titan's gravity and shape alter as the giant moon orbits its parent planet.
After calculating the likely properties of an ocean below the thick icy crust of Titan, the team was convinced that the subsurface ocean is predominantly liquid water. If true, this would offer the chance of finding extraterrestrial life.
"Solutes like ammonia will lower the density a bit, while sulphur will raise it," Professor Lunine explains. "But it isn't a layer of liquid methane — the density would be too low to account for the observed gravity data. And the overlying water ice crust would, in any event, founder if it were atop liquid methane."
It is not yet possible to peer into inside Titan, reminds CSIRO astrophysicist Kurt Liffman. So the presence of a liquid ocean is based on models of the interior. "Measurements of Titan's shape as the moon travels around Saturn can be compared with predictions from different models," Dr Liffman notes.
Some scientists believe that, in addition to vast quantities of water, Titan's ocean might also contain ammonia and perhaps a little methane.
"Based on the estimated abundances of nitrogen, hydrogen and carbon, plus the expected temperatures and pressures in the Saturnian region of the disc of gas and dust that surrounded the early sun, it is reasonable to expect the presence of ammonia and perhaps methane within Titan," Dr Liffman says.
In the view of Monash University astrophysicist Andrew Prentice, the subsurface ocean most likely consists of pure methane and beneath that a layer of ammonia ice that is quite soft.
"My model predicts a fairly thin layer of liquid methane about six kilometres deep and located 50 kilometres below Titan's outer solid crust of water-ice," Dr Prentice explains. "Beneath the liquid layer there is probably a 14-kilometre-thick layer of soft ammonia ice."
It would be very tough for any life form to survive in a subsurface ocean since there is so little sunlight that reaches the surface, Dr Prentice says. "Titan's source of internal heat through the decay of short lived radio nuclides is also very tiny."
By contrast, Jupiter's large Galilean moon Europa has a greater chance of harbouring life forms, most scientists believe.
Discovered in 1655 by Christiaan Huygens, Titan — covered in an orange haze of cloud, morning mist and naturally produced photochemical smog – has been scrutinised by telescopes and, more recently, by spacecraft.
In 2004, a small NASA probe called Huygens separated from its mother craft, Cassini. After several weeks, the probe parachuted down to Titan's surface and, for several hours, relayed close-up images to Earth.
"The Huygens probe imaged channels, but its descent through the atmosphere was at low latitudes and so it didn't detect any lakes," says Elizabeth Turtle of Johns Hopkins University's Applied Physics Laboratory in the US.
Since then, scientists have deduced that beneath the moon's clouds lurk large lakes, some spanning thousands of square kilometres. That would make them roughly the size of the Black Sea or Caspian Sea; one is thought to be bigger than Lake Superior, the world's largest freshwater lake by surface area.
"The lakes and seas are found at high latitudes and predominantly in the northern hemisphere, although there are some near the southern pole," Dr Turtle adds.
"There appear to be no permanent bodies of liquid at equatorial latitudes. However, there was once a large rainstorm over low latitudes and there may still be some areas of ponded runoff — although they've been receding."
Titan has seasons, says Candy Hansen, Cassini scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.
"That's partly why there are more lakes right now in the northern hemisphere, and why some lakes are evaporating," Dr Hansen explains. "These seasonal changes occurred as the sun has moved from the southern to the northern hemisphere in the last eight years that Cassini has been exploring Titan."
The differences between the northern and southern summers are also caused by Saturn's orbital eccentricity and possibly longer-term orbital variations, she adds.
Titan is in a state of deep freeze, with surface temperatures averaging about minus 180 degrees. If the lakes contained water they would be frozen solid. But these lakes hold liquid hydrocarbons, mainly ethane and some methane, that freeze at much lower temperatures than water.
The remote moon has something resembling Earth's water cycle, with surface liquids evaporating and returning as rain. But on Titan, it rains ethane and methane — not water — the only moon in our solar system to have such a cycle.
At Titan's South Pole, the Cassini orbiter once detected fog, caused by surface methane evaporating and then condensing into humid air.
Other likely surface features include wandering streams and riverbeds, probably carved by liquid ethane, beaches — that get more exposed as the lakes they border slowly evaporate — plains, ancient craters and volcanoes, though probably not active ones.
Weirder than weird
Is it feasible for life to exist in Titan's liquid ethane lakes? Weird life maybe, suggests Dr Prentice. But even the hardiest of microbes would struggle to tough it out, he says.
Titan appeals to life scientists, in part, because its planet-like atmosphere is denser than that of Mercury, Earth, Mars or Pluto. It is, in fact, the solar system's only moon with a thick atmosphere and a pressure about 50 per cent higher than that at sea level on Earth.
Its atmosphere consists mainly of nitrogen, the Earth's main gas — as well as hydrogen. (Some pundits say it might be similar to what the atmosphere was like here many eons ago.)
Titan is also the only place in the solar system, apart from Earth, to have liquid both on and beneath its surface.
Cassini's next flyby of Titan will be April 5, at 1400 kilometres, followed by another on May 23 at 970 kilometres. Stay tuned for more unearthly surprises.
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